1. Field of the Invention
The present invention concerns a zoom lens system system and particularly concerns a zoom lens system system applicable to a video camera or a digital camera or the like.
2. Description of the Related Art
In recent, years a compact and low cost, yet high-performance zoom lens system has been sought as a zoom lens system used in a video camera or a digital camera wherein a CCD (charge coupled device) serves as an imaging device. To attain compactness in a zoom lens system, a means is generally adopted wherein the optical power of individual lens units is augmented and the amount of motion is reduced. Higher order aberrations thereby produced are suppressed by an aspherical surface, but an aspherical surface is not capable of correcting chromatic aberration, and a lens unit requiring correction of chromatic aberration requires at least two lenses.
At the same time CCD format size has experienced an ongoing trend of miniaturization, and miniaturization of overall lens size has consequently been sought. Yet when lenses are made smaller overall, sensitivity to accompanying manufacturing errors increases readily. Particularly in lens units comprising multiple lenses, there is an intensified sensitivity to degraded lens performance arising from air gap errors and decentering errors. Thus, if a lens unit could be constructed from a smaller number of lenses, not only could compactness be achieved effectively, the ability to manufacture more advantageously would allow cost reduction. Yet as discussed above, an aspherical surface is incapable of correcting chromatic aberration, which imposes a limit on using as few lenses as possible to construct a lens unit requiring correction of chromatic aberration.
In addition to an aspherical surface, the use of a refractive index distribution-type lens can reduce the number of lenses used in construction. A refractive index distribution-type lens has the ability to correct all aberrations including chromatic aberration and has a major effect on reducing the number of lenses used in construction. Yet in nearly all cases, an attempt to correct chromatic aberration in a design embodiment, by using a refractive index distribution-type lens results in dispersion and distribution in a negative direction, which is extremely problematic in manufacturing. As described above, it is extremely difficult to correct chromatic aberration through a construction with a small number of lenses.
It is a known fact, that a diffracting optical unit, has the ability to correct chromatic aberration, and there are numerous proposals to correct, the chromatic aberration of a lens used in a photographic camera or a CD (compact, disc) pickup by means of a combination of a diffracting optical unit and a refracting lens. For example, Japanese Laid-Open Patent Application No. 6-324262 discloses the application of a diffracting optical unit to an imaging lens. Nevertheless there is as yet no known zoom lens system with performance high enough to allow use in a camera employing a CCD or other such imaging unit and which is at the same time compact and low cost.
Japanese Laid-Open Patent Application No. 5-164965 and others propose a negative/positive two-component zoom lens systems and therein is a known mobile lens unit constructed from one refracting lens. Each two-component zoom lens system is nevertheless a zoom lens system with a magnification rate low enough for inexpensive applications such as lens-affixed films and does not attain performance satisfactory for a zoom lens system used in a camera employing a CCD or other such imaging unit.